Printing systems such as copiers, printers, facsimile devices or other systems having a print engine for creating visual images, graphics, texts, etc. on a page or other printable medium typically include various media feeding systems for introducing original image media or printable media into the system. Examples include direct thermal printers and thermal transfer printers. For thermal transfer printing on nonsensitized materials such as paper or plastics, a transfer ribbon coated on one side with a heat-transferable ink layer is interposed between the media to be printed and a thermal printhead having a line of very small heater elements. When an electrical pulse is applied to a selected subset of the heater elements, localized melting and transfer of the ink to the paper occurs underneath the selected elements, resulting in a corresponding line of dots being transferred to the media surface.
After each line of dots is printed, the material or printhead is repositioned to locate and print on an adjacent location, the transfer ribbon is repositioned to provide a replenished ink coating, and the selecting and heating process is repeated to print an adjacent line of dots. Depending upon the number and pattern of heaters and the directions of motion of the head and paper, arrays of dots can produce individual characters or, as in the preferred embodiment, successive rows of dots are combined to form complete printed lines of text, bar codes, or graphics.
For direct thermal printing, a heat sensitive media is used along with a thermal printhead having a line of very small heater elements . When an electrical pulse is applied to a selected subset of the heater elements, a thermal reaction to the heat sensitive media occurs underneath the selected elements, resulting in a corresponding line of dots being printed on the media surface.
Applications of such printers include the printing of individual labels, typically pressure-sensitive labels, tickets, and tags. Pressure-sensitive labels are commonly presented on a continuous web of release material (e.g., waxed paper backing) with a gap between successive labels. Tickets and tags may likewise be presented as a continuous web with individual tickets or tags defined by a printed mark or by holes or notches punched therein. Tickets and tags also may likewise be presented on a continuous web with individual tickets or tags defined by a printed mark or by holes, slits, or gaps punched therein.
Such printers also may be adapted to permit the removal of individual labels as they are printed. The construction of the printhead may be such that the web and ribbon are advanced by the length of the inter-label gap plus a significant fraction of an inch after printing of each label and before stopping for removal of the label, in which case the web and ribbon must be backfed an equal distance before printing the next label to avoid leaving an unprintable area of the label.
The power flow to each heater element during energization is relatively constant, being determined by the supply voltage and the electrical resistance of the heater. The energy per printed dot for uniform ink transfer is a function of the web speed and the average printhead temperature. When printing individual labels, the web speed may not be constant, but may be smoothly accelerated and decelerated to allow for inertia of the mechanism. This requires changes in the energization to maintain uniform print quality across the areas printed during speed changes.
Such printers should complete the individual labels as rapidly as practical upon receipt of data therefor. Printing of a label requires three steps: receipt by the controller of a label description in a terse label-description language describing the known objects to be printed, such as text and bar codes but not the dot patterns from which they are formed; formation of the label image in a bit-map memory by the controller, where bits in the map correspond to physical dots in the image; and transfer of the dots forming the label image from bit-map to the printhead, energization of the printhead, and feeding of the web and transfer ribbon as described above. The thermal transfer ribbon may be fed from a supply roll before printing and then taken up on a take-up spindle after use.
Conventional direct thermal printers and thermal transfer printers typically require a manual adjustment of printhead pressure prior to or during the printing operation. This manual adjustment is typically performed via a screw or knob located about the printer's housing and connected to a biasing mechanism affixed to the printhead. Undesirably, these printers have no means of continuously determining and adjusting the pressure of the printhead in proportion to the thickness of media passing thereunder during a printing operation. As a result, the printhead pressure may increase or decrease to undesirable levels. One skilled in the art will appreciate that too much printhead pressure may result in physical wear on the printhead causing failure; while too little pressure on the printhead may result in undesirably light print being transferred onto the media. It would therefore be desirable to provide an apparatus and method operable for determining and adjusting the printhead pressure during a printing operation to provide optimal print quality. It would also be desirable to provide a sensing mechanism connected to the printhead and adapted for determining a set of conditions and communicating with a control circuit operable for controlling a motor drive which can adjust the pressure to pre-defined levels. In addition, the use of conventional thermal transfer printers and more specifically the manual adjustment of the printhead pressure does not account for or maintain a center bias. Thus, there exists a need to utilize a printhead that is operable for adjustment in a synchronized manner thereby maintaining a center bias.